1. Field of the Invention
The present invention relates to an antifuse, and particularly to an antifuse which improves on-state reliability.
2. Description of the Related Art
Antifuses are well known in the art of programmable logic devices. An antifuse is a structure which is non-conductive when manufactured, but becomes permanently conductive by applying a predetermined voltage across its terminals (i.e. programming the antifuse). Antifuses are typically used in programmable logic devices to programmably interconnect conductive lines.
FIG. 1 illustrates a cross-section of a typical antifuse 110 having a substrate 100, a first insulating layer 101 formed on substrate 100, and a first conductive layer 102 formed on first insulating layer 101. Typically, first conductive layer 102 includes aluminum. A first barrier metal layer 103 is formed and patterned on first conductive layer 102 (wherein first barrier metal layer 103 and first conductive layer 102 in combination are referred to as the bottom electrode). A second insulating layer 104 is deposited and etched to form a via to barrier metal layer 103. An amorphous silicon layer 105 is formed over second insulating layer 104 and first barrier metal layer 103. A second barrier metal layer 106 as well as a second conductive layer 107 are formed on amorphous silicon layer 105 (wherein second barrier metal layer 106 and second conductive layer 107 in combination are referred to as the top electrode).
During programming, a conductive link (not shown) is formed between first barrier metal layer 103 and second barrier metal layer 106 by applying a positive voltage to either the top or the bottom electrode (referenced herein as the positive electrode). The electrode not receiving the positive voltage (i.e. the negative electrode) becomes heated and melts the associated barrier metal layer through the amorphous silicon layer, thereby forming the conductive link. For additional information regarding the formation of the conductive link, see "Antifuse Reliability and Link Formation Models", A. Iranmanesh et al., 1994 International Integrated Reliability Workshop Final Report, IEEE, which is incorporated herein by reference. Note that barrier metal layers 103 and 106 prevent the interdiffusion of the aluminum with the silicon in amorphous silicon layer 105.
As known by those skilled in the art, this conductive link has an associated resistance. Typically, this resistance increases somewhat over time. This phenomena is referred to herein as R.sub.on aging. If the resistance is too high, the functioning of the antifuse may be adversely affected, even resulting in failure. In a programmable logic device, the failure of one antifuse may be disastrous since the programmed circuit may no longer be realized in the integrated circuit. Thus, a goal of any antifuse process is to improve R.sub.on aging as much as possible. Therefore, a need arises to provide a method of improving R.sub.on aging, thereby ensuring greater on-state reliability of the antifuses.